1. Field of the Invention
The present invention relates to a communication terminal apparatus, communication system, communication method, and program, and more particularly to improvement of an error control technology suitable for communication in an environment prone to the occurrence of code errors, such as mobile radio communication.
2. Description of the Related Art
Error correction coding technology and automatic retransmission control technology (ARQ: Automatic Repeat Request) are important error control technologies in performing digital communication. A technology named Hybrid ARQ that uses both of these technologies is known to be extremely effective particularly in a fading environment such as in the case of mobile radio communication.
An outline of the operation of a mobile communication apparatus in packet communication using Hybrid ARQ will now be given with reference to FIG. 1. Details of the radio communication method are given in 3GPP (3rd Generation Partnership Project), for example.
FIG. 1 is a block diagram showing the configuration of a conventional radio communication system.
In FIG. 1, a radio communication system is composed of a base station (BTS) 10 and a plurality of mobile communication apparatuses 20.
A mobile communication apparatus 20 is configured by means of a radio antenna 21, a demodulation section 22, a data combining section 23, a decoding section 24, an error correction section 25, an error determination section 26, a control section 27 implemented by hardware (HW) or software (SW), and a transmitting section 28.
In the above-described configuration, demodulation section 22 in mobile communication apparatus 20 receives packet data transmitted from base station 10, and performs demodulation processing. Data combining section 23 performs combining processing on past demodulated packet data and current demodulated packet data. Decoding section 24 decodes the combined data and outputs decoded data to error correction section 25. Error correction section 25 performs error correction on the decoded data, and error determination section 26 performs error determination processing on the error-corrected data. If there are no errors in the data, an ACK (Acknowledgment) signal is transmitted to base station 10 from transmitting section 28. Base station 10 receives the ACK signal from mobile communication apparatus 20, and transmits new data to mobile communication apparatus 20.
On the other hand, if the packet data is determined to contain an error as a result of the error determination processing, mobile communication apparatus 20 transmits a NACK (Negative Acknowledgment) signal to base station 10 from transmitting section 28, and requests retransmission. Mobile communication apparatus 20 performs demodulation processing, data combining processing, decoding processing, error correction processing, and error determination processing again on data retransmitted from base station 10.
The above-described processing is performed until error determination section 26 determines that no error is contained in the relevant packet data. That is to say, each time a NACK signal is returned by mobile communication apparatus 20, base station 10 transmits the same packet data to mobile communication apparatus 20, selecting the optimal modulation method for the radio environment, and mobile communication apparatus 20 repeatedly performs demodulation processing, data combining processing, decoding processing, error correction processing, and error determination processing.
In this case, in an adverse fading environment, an error correction code is ineffective because of burst errors and there are many retransmission requests, and a mobile communication apparatus must repeat the series of demodulation processing, data combining processing, decoding processing, error correction processing, and error determination processing operations each time retransmission is performed from the base station. This results in extremely wasteful power consumption by the mobile communication apparatus, affecting the standby time and call time of the mobile communication apparatus.
In order to solve this problem, there is a technology whereby data received by a mobile communication apparatus and previously received data are combined, quality is found from the result of the combining operation, and the procedure is repeated until a predetermined quality level is attained (see, for example, Patent Document 1: National Publication of International Patent Application No. 2002-523934).
However, a problem with this kind of conventional radio communication system is that, if the radio communication environment created by a mobile communication apparatus and base station is poor, data reception, combining processing, decoding processing, error correction processing, and error determination processing are performed repeatedly, consuming power.
For example, in Patent Document 1, although the data error rate when decoding processing is performed falls significantly since combining processing is performed on previously received data and newly received data until data received by a mobile communication apparatus attains a predetermined quality level, no mention is made of the control of the decoding section until the predetermined quality level is attained. As the packet data transmission speed increases, it becomes necessary for a plurality of decoding sections to be provided in a mobile communication apparatus, and for these to perform processing in parallel. Therefore, the wasteful mobile communication apparatus power consumption will be fully appreciated by those skilled in the art.